The heat released by an electrical arc has long been used for melting and fusion for joining metals together. Processes termed percussion, flash and projection welding are fundamentally similar in that the electrical arc which releases required energy is made to take place directly between the two metals to be joined. The sense of polarity (in the case of direct current) and the rise, duration and fall time of current flow are factors most often employed to control melting of the abutting surfaces before impact of the two members to be joined together and subsequent cooling which forms the weld.
Problems have been encountered in utilizing the foregoing processes with members of certain configurations and of certian metals or alloys relative to obtaining a molten surface on both of the members which are to be joined together. Due to the nature of the arcing process considerably more heat is liberated at the positive or anode surface of a member than at the cathode surface so that, even when the metals being joined are similar, uneven heating can result to produce a weld with degraded properties. Also in certain instances, it is neither practical nor desirable to make electrical contact with one of the members to be joined. An example is in the case of welding an enameled copper conductor to some electrical fitting. To make electrical contact to the copper conductor by stripping the enamel is both time consuming and costly and piercing the enamel coating usually results in damage to the conductor.
When metals to be joined together have widely different electrical and heat conductivities, it is very difficult to control direct arcing between them so as to control temperature rise and melting of the metals with present day processes. There are many instances applicable to both consumable and non-consumable electrode arc welding where the introduction of a new and fresh electrode surface for each welding cycle would offer substantial advantages. This is especially true for those processes utilizing short duration, pulsed arc techniques wherein it would be desirable to reproduce arching conditions from one cycle to the next regardless of the repetition rate at which the welds are made. Unless the members to be welded together are preconditioned or treated the introduction of precise amounts of fluxes, antioxidants, protective coatings and arc modifying agents like those used with coated rod type cathodes in consumable electrode welding cannot be readily accomplished in percussion, flash or projection type welding.